1. Field of the Invention
Invention relates to apparatus for measuring the effectiveness of a photographic developer processes and more particular to apparatus for testing and optimizing the photographic film developing process with respect to a predefined standard.
2. Background Art
In diagnostic radiology, medical practitioners rely on the sensitivity and accuracy of the radiographic image in formulating a medical diagnosis. There are many variables, however, in the production of a radiographic image which can lead to incomplete or improper diagnosis. As in any photographic process, the x-ray image is formed on a light sensitive film by a controlled exposure. Typically, the x-ray sensitive film is an acetate cellulose base film coated with an emulsion of silver halide and gelatin. The film may be placed between a pair of x-ray activated phosphorous screens which are responsive to x-ray energy to emit light of a particular color to enhance exposure of the film.
In diagnostic radiology, an object such as a limb or other portion of the body to be diagnosed is placed between an x-ray tube and the photographic film. As x-rays pass through the object, shadow images of areas of varying density are formed on the photographic film representing bone, tumors, or the like. After the film has been exposed in the x-ray process, it is developed and subsequently interpreted by a medical professional.
Proper x-ray interpretation in the diagnosis process depends, to a large degree, on the accuracy of the finished photograph. The clarity and the visible distinction between various portions of the image depend on the level of energy applied by the x-ray tube as well as on the sensitivity of the film and the characteristics of the film developing process. The exposed film is typically developed in an automatic processing device referred to as a developer processor. There are several variables in the developing process which may change over time and which effect the contrast in the developed film. Such variables include the temperature, the chemistry of liquids in the processor, the speed at which the film is advanced through the processor, and the like.
A problem with the prior art arrangement is that there are no independent standards defining control of automatic film developer processors to ensure the quality of the processed images in medical radiographic films. However, there is an increased awareness of the importance of proper film development and a need for measuring and controlling the process. In current practice, the radiologist or a skilled x-ray technician decides when the processor is operating at a level which is acceptable to produce an image of acceptable clarity. Clarity of the image can often be improved by increasing the level of energy produced by the x-ray tube. However, for the protection of the patient, the level of radiation should be kept to a minimum and maximum allowable energy levels are often specified for the equipment. It is therefore desirable to optimize the film developer processor and, particularly, to be able to define a standard of optimization for the processor.
In a known method of comparing the results of a processor with a previous result, a strip of test film is exposed by means of an instrument known as a sensitometer which includes a stable light source and a transparency-gradient step wedge plate. A typical step wedge plate provides a graduated series of 21 exposed areas ranging from full exposure to essentially no exposure. The test strip is developed in a well-adjusted processor and the density values of the separate exposed areas are measured by a well-known densitometer. These density values and several quality control parameters derived from them, e.g. speed index, base and fog, contrast index, average gradient, are recorded. At a later time, a test film strip is again exposed by means of the sensitometer, developed in the processor, and tested by means of the densitometer. The new densitometer readings are compared with the previously recorded values. If the deviations of any of the parameters exceeds predetermined limits, the cause of the deviation is investigated and, if necessary, the automatic developing processor is adjusted until an acceptable operating level is again reached.
Since process control is used only to maintain an automatic film developer processor at a particular operating level after it has once been determined to be at an acceptable operating level, the primary requirement of the sensitometer is that the instrument provide consistent and repeatable exposure. This is critical because variations in the density values of the test strips are assumed to be caused by variations in the developer processor due to variations in the chemistry, temperature, feed rate, etc. A tight inter-instrument agreement specification between sensitometers is not required since the control of any one automatic film developer processor is specific to the sensitometer used in setting up the processor.
A well known Density versus Relative Log Exposure curve is depicted in FIG. 1. This curve is derived from density readings from the steps of a step wedge exposed film. Density values are plotted along the abscissa and relative log exposure values are plotted along the axis. The Straightest portion of the curve is in the area corresponding to step 11 of the step wedge. Readings in this area are typically chosen to compute the contrast index, speed index, and other parameters. The computations are based on the assumption that the curve varies regularly over a distance covering several steps and that the level of exposure of a step varies by a factor of 1.4125 (.sqroot.2) from adjacent steps. Thus, on the relative log exposure scale adjacent steps are separated by log .sqroot.2=0.150. Inaccuracies, however, are introduced since, in actual practice, the steps do not vary regularly in .sqroot.2 increments. This may be due to variations in tolerances in the step wedge filter used in the sensitometer as well as variations in illumination over the lengths of the step wedge in the sensitometer. Thus, indices computed on the basis of readings from a number of wedge steps have inherent inaccuracies.
In the continuing effort to provide the best possible image quality while minimizing the level of x-ray radiation to which a patient is exposed, an effort is currently underway to allow processors to be optimized for specific radiographic film and specific developer chemistry. Thus, it is desirable to provide x-ray equipment operators with the tools and information required to obtain optimum film performance for a specific film/chemistry/processor combination.